Spectator exciton effects in nanocrystals III: Unveiling the stimulated emission cross section in quantum confined CsPbBr3 nanocrystals

Quantifying stimulated emission in semiconductor nanocrystals (NCs) remains challenging due to masking of its effects on pump-probe spectra by excited state absorption and ground state bleaching signals. Absence of this defining photo-physical parameter in turn impedes assignment of band edge electronic structure in many of these important fluorophores. Here we employ a generally applicable 3-pulse ultrafast spectroscopic method coined the “Spectator Exciton” (SX) approach to measure stimulated-emission efficiency in quantum confined inorganic perovskite CsPbBr3 NCs, band edge electronic structure of which is the subject of lively ongoing debate. Our results show that in 5-6 nm CsPbBr3 NCs, a single exciton bleaches more than half of the intense band edge absorption band, while the cross section for stimulated emission from the same state is nearly 6 times weaker. Discussion of these findings in light of several recent electronic structure models for this material proves them unable of simultaneously explaining both measures. proving the importance of this new input to resolving this debate. Along with femtosecond time resolved photoluminescence measurements on the same sample, SX results also verifies that biexciton interaction energy is intensely attractive with a magnitude of ~80 meV. In light of this observation, our previous suggestion that bi-exciton interaction is repulsive is reassigned to hot phonon induced slowdown of carrier relaxation leading to direct Auger recombination from an excited state. The mechanism behind the extreme slowing of carrier cooling after several stages of exciton recombination remains to be determined.